Synthesis and evaluation of nitroheterocyclic carbamate prodrugs for use with nitroreductase-mediated gene-directed enzyme prodrug therapy

A variety of nitroheterocyclic carbamate prodrugs of phenylenediamine mustard and 5-amino-1-(chloromethyl)-3-[(5,6,7-trimethoxyindol-2-yl)carbonyl]-1,2-dihydro-3H-benz[e]indoline (amino-seco-CBI-TMI), covering a wide range of reduction potential, were prepared and evaluated for use in gene-directed enzyme prodrug therapy (GDEPT) using a two-electron nitroreductase (NTR) from Escherichia coli B. The carbamate prodrugs and corresponding amine effectors were tested in a cell line panel comprising parental and NTR-transfected human (SKOV3/SKOV3-NTR(neo), WiDr/WiDr-NTR(neo)), Chinese hamster (V79(puro)/V79-NTR(puro)), and murine (EMT6/EMT6-NTR(puro)) cell line pairs and were compared with the established NTR substrates CB1954 (an aziridinyl dinitrobenzamide) and the analogous dibromomustard. The 1-methyl-2-nitroimidazol-5-ylmethyl carbamate of phenylenediamine mustard was metabolized rapidly by EMT6-NTR(neo) but not EMT6 cells, demonstrating that it is an efficient substrates for NTR. Despite this, the carbamates of phenylenediamine mustards show relatively low differential cytotoxicity for NTR+ve cells in IC(50) assays, apparently because they retain sufficient alkylating reactivity that most of the prodrug reacts with nucleophiles during the drug exposure period. In contrast, the corresponding amino-seco-CBI-TMI prodrugs were less efficient NTR substrates but had greater chemical stability, were more potent, and showed substantial NTR-ve/NTR+ve ratios in the cell line panel, with ratios of 15-100-fold for the 1-methyl-2-nitro-1H-imidazol-5-ylmethyl and 1-methyl-5-nitro-1H-imidazol-2-ylmethyl carbamates of amino-seco-CBI-TMI. The activity of these two prodrugs was evaluated against NTR-expressing EMT6 tumors comprising ca. 10% NTR+ve cells. Small but not statistically significant killing of NTR+ve cells was observed, with no effect against NTR-ve target cells. The lack of activity against NTR+ve cells in tumors, despite potent and selective activity in culture, indicates that pharmacokinetic optimization will be required if in vivo efficacy against solid tumors is to be achieved with this new class of NTR prodrugs.